Recently, there have been widely in use rubber-reinforced resins improved in impact strength through the blending with a graft copolymer obtained from the graft polymerization of a rubber polymer with a monomer capable of imparting compatibility with hitherto known thermoplastic resins such as polyvinyl chlorides, polystyrenes, polymethyl methacrylates, styrene-acrylonitrile copolymer resins, .alpha.-methylstyrene-acrylonitrile copolymer resins, styrene-acrylonitrile-phenylmaleimide copolymer resins, polyester resins, polycarbonate resins and polyamide resins, and polymer alloys of these resins such as alloys of styrene-acrylonitrile copolymer resins with polycarbonate resins and alloys of .alpha.-methylstyrene-acrylonitrile copolymer resins with vinyl chloride resins.
The rubber copolymer used for the preparation of such rubber-reinforced resins has an optimum range of particle size, which is generally known to be dependent on the kind of a matrix resin. It is said that a kind of rubber larger in particle size is required for polymers higher in brittleness and actually the average particle sizes of the rubbers in use range from 0.15 .mu.m (150 nm) to several .mu.m (several thousands nm).
Of the various kinds of rubbers used for the purpose, most widely in use are of diene type or of acrylic ester type.
The diene-type rubbers and acrylic ester-type rubbers are normally prepared by emulsion polymerization and are obtainable in latex form. The particle size of the rubbers obtained by emulsion polymerization is not more than 0.1 .mu.m (100 nm) unless a special treatment is given for the purpose, this being too small for a graft polymer as a material of a rubber-reinforced resin. Various methods have, therefore, been practiced or proposed for increasing the particle size of a rubber latex to the desired level. One is an approach to improve the polymerization method for increasing the particle size of the rubber latex, and the other is an approach to increasing the particle size of the rubber latex through the cohesion of small particle size rubber prepared by usual emulsion polymerization.
The former approach is for increasing the particle size in the course of polymerization under conditions of, for example, high polymer concentration and high-shear stirring for growth by the cohesion of rubber particle size. The greatest demerit of this method is the too long polymerization time, 50-100 hours for increasing the particle size to about 0.3 .mu.m (300 nm), and this extremely low productivity can hardly be acceptable commercially.
The latter approach is for increasing the particle size through the cohesion of particles by lowering the stability of a rubber latex through the addition of some inorganic salt or acid. The particle size attainable by this method is 0.2 .mu.m (200 nm) at largest, this being not proper for rubber-reinforced resins in most cases. An attempt to further increase the particle size by this method results in the formation of a large amount of clots, this being difficult to be practiced commercially.
An improvement of this method is disclosed in Japanese Laid-open Patent Publication No. 25655/'75. It is a method for increasing the particle size by adding to a rubber latex of not less than pH 7, an acid group-containing latex with its core made of a polyalkyl acrylate and its shell made of a copolymer of an alkyl acrylate and an unsaturated acid. By this method it is, indeed, possible to make rubbers large in particle size, 0.3 .mu.m (300 nm) or so.
This method, however, caused the formation of a large amount of clots during the preparation of the acid group-containing latex, and the resultant lowering of production efficiency and troubles of disposal of the resulting wastes were problematic for commercial adoption thereof.
Lowering of the content of unsaturated acids for suppressing clot formation during the preparation of an acid group-containing latex itself results in lowering of a particle-growing potential, which gave rise to problems such as failure to increase the rubber particle size, increase of ungrown particles and deterioration of mechanical properties of thermoplastic resins.
After intensive studies for solving the aforementioned problems about the method of growing particles by the use of an acid group-containing latex the present inventors have found out that an acid group-containing latex of a specific composition of an unsaturated acid and an alkyl methacrylate reduces clot formation during the preparation, provides rubbers of large particle size and the rubber-reinforced thermoplastic resin made by the use thereof has a high impact strength, and thus arrived at the present invention.